Traditionally, sintered inorganic materials such as ceramics, metals or composites thereof (cermets) have been formed by molding a powder of the material into a desired three dimensional shape, recovering the shape from the mold and subsequently firing the shape. In the traditional sintering process, much expense goes into the molding process. For example, in injection molding, a suitable mold must first be created from a material capable of withstanding the injection molding process. In pressing or extrusion processes, appropriate dies must be selected or made by machining. The traditional molding processes generally require substantial capital investment in equipment to produce molded parts on an industrial scale.
Some attention has been given toward efforts to eliminate the need for molding in the formation of three dimensional parts. Various processes are discussed in U.S. Pat. Nos. 5,207,371 and 5,126,529. Japanese Laid-Open Patent Application 04-099,203 discusses the use of photocurable organic materials to make ceramic or metallic structures by what appears to be a photolithographic process, but the Application does not give any detail as to how this is to be achieved in a stereo photolithography (also called "stereolithography") process.
Stereo photolithography is a method which has been used for making prototype plastic parts. This method is discussed in U.S. Pat. No. 4,575,330, the disclosure of which is incorporated herein by reference. Stereo photolithography involves the formation of a thin layer of liquid photopolymerizable on a substrate. Once the layer is formed, it is selectively exposed to curing radiation (typically UV) via a scanning laser. Once the desired portion of the liquid layer is cured an additional liquid layer is flowed over the first cured layer. The second liquid layer is then scanned with the curing laser to form a second layer of cured photopolymer. This process is repeated a multitude of times to build up a cured polymer body. Typically, the laser is computer programmed to scan the layer-by-layer patterns corresponding to the desired shape.
In a typical stereo photolithography apparatus, the substrate is immersed in a bath of the photopolymerizable organic liquid such that the top surface of the substrate is covered with a thin layer of the liquid. Once the thin layer is scanned to cure the desired regions (while the substrate is still in the bath), the substrate is lowered deeper into the bath whereby additional photopolymerizable organic liquid flows over the first cured layer to form another thin layer of liquid which is then scanned with the laser. The lowering of the substrate into the photopolymerizable liquid can also be computer programmed such that virtually the entire stereo photolithography process can be automated. In this way, stereo photolithography has been capable of producing intricate prototype plastic parts in a matter of hours without the need for mold making or machining.
The use of photocuring techniques such as stereo photolithography has largely been limited by the need for substantial flowability of the photocuring material. As a result, this technique has not been used to make dense sintered parts of inorganic materials.